This invention relates in general to hermetic sealing of semiconductor chips, and more particularly, to a ceramic lid assembly having a selectively deposited integral heat fusible layer for hermetic sealing of a package containing a semiconductor chip.
Integrated circuits in the form of one or more semiconductor chips are assembled for utilization in small packages generally known as flat packs. The flat pack encloses the semiconductor chip for protection of the various components therein to prevent damage from handling as well as atmospheric and other destructive conditions. The flat pack is formed of a package having a central opening on one major surface. The semiconductor chip is placed within a cavity of the flat pack defined by the central opening and is connected electrically by leads extending through the walls of the package. It has become conventional to provide a hermetically sealed cover over the central opening to protect the semiconductor chip.
Heretofore, it has been the practice to hermetically seal the semiconductor chip within the cavity of the flat pack by placing a preformed ring of heat fusible material, such as a gold-tin eutectic solder, on the sealing area of the package surrounding the cavity, and overlying the central opening with a gold placed Kovar lid. Upon heating the thus formed assembly, the gold-tin eutectic solder forms a hermetic seal between the package and the gold plated lid.
The solder materials of the preforms are generally brittle and their dimensions are so small that the preforms are very fragile and extremely difficult to handle during assembly. Because of the difficulty of handling such preforms, it has also been difficult to insure accurate registration between the preform, the peripheral region of the lid and the sealing area surrounding the cavity of the package. As a consequence, there has been a substantial yield loss in the assembled flat packs due to defects in the hermetic seal. In addition, with the price of gold increasing to unforeseen limits, there is a real economic need to minimize or completely eliminate the use of gold in providing a hermetically sealed flat pack.
One hermetic sealing technique which partially overcomes the above noted disadvantages is described in U.S. Pat. No. 3,874,549. In general, a gold plated Kovar lid for hermetic sealing of the package is superimposed with a heat fusible preform. The precise registration between the preform and the lid is achieved by spot welding the preform at a plurality of space locations about the peripheral area of the lid.
However, the welding of the preform to the lid greatly increases the risk of trapping dirt and dust therebetween which ultimately causes leaks in the hermetic seal. Visual inspection of the welded preform to the lid will not give any indication of the subsequent quality of the hermetic seal. Accordingly, defects in the hermetic seal are not noticed until the flat pack assembly is fully fabricated, resulting in an additional loss in yield. As these lids with welded preforms are comingled in automatic processing equipment, there is a tendency for the corners of the preforms to lift up and break off, further reducing the overall yield and quality of the hermetic seal.
With the semiconductor chip mounted in the cavity of the package, electrical connections must be made from the terminal pads on the semiconductor chip to the leads extending through the package walls. The most widely used method is thermo-compression bonding. Gold wire about one mil in diameter is used in a process called Ball Bonding. The gold wire, if not precisely aligned within the cavity of the flat pack, is in danger of making contact with the conductive gold plating on the Kovar lid. In the event of contact by the gold wires with the conductive gold plated lid, the semiconductor chip becomes shorted and inoperative. The use of a dielectric lid results in difficulty in obtaining a hermetic seal as it is known that the gold-tin eutectic solder used to fabricate the preforms will not wet and form a hermetic seal to dielectric materials such as ceramics.
Thus, there is heretofore an unsolved need to provide a dielectric lid for use in hermetically sealing semiconductor devices which overcomes the above-noted disadvantages resulting in the use of known lids and hermetic sealing techniques.